1. Technical Field
The present disclosure relates to a microelectromechanical device with position-control driving and to a method for controlling a microelectromechanical device.
2. Description of the Related Art
As is known, the use of microelectromechanical systems (MEMS) has become increasingly widespread in various fields of technology and has yielded encouraging results especially in the construction of inertial sensors, microintegrated gyroscopes, and electromechanical oscillators for a wide range of applications.
MEMS of this type are usually based upon microelectromechanical structures comprising at least one movable mass connected to a fixed body (stator) through springs and movable with respect to the stator according to pre-determined degrees of freedom. The movable mass is moreover coupled to the fixed body via capacitive structures (capacitors). The movement of the movable mass with respect to the fixed body, for example on account of an external stress, modifies the capacitance of the capacitors, whence the possibility of getting back to the relative displacement of the movable mass with respect to the fixed body and hence back to the force applied. Vice versa, by supplying appropriate biasing voltages, it is possible to apply an electrostatic force to the movable mass to set it in motion. In addition, to produce electromechanical oscillators the frequency response of the inertial MEMS structures is exploited, which is typically of the second-order low-pass type.
Many MEMS (in particular, all the electromechanical oscillators and gyroscopes) must envisage driving devices that have the task of maintaining the movable mass in oscillation.
A first type of known solution envisages applying, in open loop, periodic stresses at the resonance frequency of the MEMS structure. The solution is simple, but also far from effective, because the resonance frequency is not known precisely on account of the non-eliminable spreads in semiconductor-micromachining processes. In addition, the resonance frequency of each individual device can vary over time, for example on account of temperature gradients or, more simply, of ageing.
Feedback driving circuits, based upon the use of sigma-delta modulators, have then been proposed. Circuits of this type are undoubtedly more effective than the previous ones in stabilizing the oscillation of the movable mass at the real resonance frequency and in suppressing disturbance. However, various stages are necessary for filtering, decimation, and further processing of the bitstream supplied by the sigma-delta modulator. For this reason, currently available feedback driving circuits are complex to produce, cumbersome and, in practice, costly.
In addition, it must be taken into account that gyroscopes have a complex electromechanical structure, which comprises two masses that are movable with respect to the stator and coupled to one another so as to have one relative degree of freedom. The two movable masses are both capacitively coupled to the stator. One of the movable masses is dedicated to driving (driving mass) and is kept in oscillation at the resonance frequency. The other movable mass (sensing mass) is drawn along in the oscillatory motion and, in the case of rotation of the microstructure with respect to a pre-set axis with an angular velocity, is subject to a Coriolis force proportional to the angular velocity itself. In practice, the sensing mass functions as an accelerometer that enables sensing of the Coriolis acceleration.
In order to enable driving and provide an electromechanical oscillator in which the sensor performs the role of frequency-selective amplifier, with second-order transfer function of a low-pass type and high figure of merit, the driving mass is provided with two types of differential capacitive structures: driving electrodes and drive-sensing electrodes. The driving electrodes have the purpose of supporting the self-oscillation of the movable mass in the driving direction, through electrostatic forces generated by the spectral component of the noise at the mechanical resonance frequency of the driving mass. The drive-sensing electrodes have the purpose of measuring, through the charge transduced, the position of translation or rotation of the sensing mass in the driving direction.
U.S. Pat. No. 7,305,880, which is incorporated by reference herein in its entirety, describes a system for controlling the velocity of oscillation of the gyroscope, which comprises a differential read amplifier, a high-pass amplifier, and a driving and control stage, operating in continuous-time mode. This system, albeit operating satisfactorily, may however undergo improvements as regards its overall dimensions.
U.S. Application Publication No. 2008/0190198, which is incorporated by reference herein in its entirety, describes an improvement of the above control system, in which the control loop comprises a low-pass filter having the purpose of reducing the offset and effects of parasitic components and couplings, operating on the overall gain and phase of the feedback loop.